The present invention relates to optical components, which influence the polarization state of incident polarized light. More in particular transmissive phase retarding elements with reduced dimensions and enhanced functionality for large laser beam applications are the subject of this invention.
Phase retarders are optical components, which shift the phase between the two polarization components of an incident polarized laser beam. The introduction of a 90 degree phase shift is generally described as a linear-to-circular transformation. A phase shift of 180 degrees is typically described as changing the rotation of linearly polarized light. Phase retarding elements can be subdivided in reflecting and transmissive types. As the names imply, with reflecting phase retarders, the incident light is reflected back from the retarder element with a shifted phase, whereas with transmissive types, the phase shift is introduced as the incident light passes through the element.
For increased efficiency, phase retarders are required to have a high optical throughput. For a reflecting type retarder, the reflectivity of the surface should be as high as possible. For a transmissive phase retarder, the transmission of light through the element should be as high as possible.
For high power laser applications, the phase retarder needs to be able to withstand the high optical power of the incident laser beam. Additionally, high power laser applications typically involve a relatively large diameter laser beam. A typical beam in CO2 laser optics can be from several millimeters up to several centimeters in diameter. Prior art phase retarders have needed to be commensurately large, resulting in bulky and expensive solutions in the past.
Prior Art 90 Degrees Phase Retarders
FIG. 1 illustrates a prior art phase retarder. The illustrated optical arrangement provides for a quarter wave plate (90 degrees phase shift) using a copper mirror 5 that operates in reflection mode at mid- or far-infrared (IR) wavelengths. Mid- and far-IR wavelengths typically range from 2-20 xcexcm. Mirror 5 is comprised of metal substrate 1, upon which is coated multiple thin film layer 2. Metal substrate 1 is used because metals generally provide a high reflection coefficient. The multilayer coating 2 provides the phase shifting function. In the illustrated example, coating 2 is designed such that a laser beam 3 incident on the element surface at an angle of 45 degrees is converted into a reflected beam 4 at the same angle. The shift between the two polarized components of incident beam 3 is shifted by 90 degrees when the incident light is linearly polarized under an angle of 45 degrees with respect to the input plane of mirror 5. Note that the operation of the illustrated prior art phase retarder requires a redirection of the propagation path of the incident light beam 3. This is disadvantageous because of the need for critical alignment of various optical elements using such devices. Any adjustment of one optical element, such as the illustrated phase retarder, would result in a misalignment of all the other optical components of a system. If yet another polarization state of the infrared beam is desired (requiring yet another phase retarder), the complete system would need to be redesigned to allow for another optical clement introducing yet another redirection of the propagation path. Hence the prior art reflection type phase retarders have the disadvantage of requiring critical alignment between elements and make optical system design and redesign difficult.
When more flexibility is required in an optical arrangement, it is preferable that the phase retarder operates in transmission mode, to avoid the above discussed disadvantages. One prior art approach to transmission mode phase retarders uses birefringent materials. These types of retarders exploit the dependency of the refractive index of orientation of the polarization components of the incoming light beam. A phase shift is introduced between the polarization state aligned with the fast axis of the birefringent material with the smallest refractive index and the polarization state aligned with the slow axis, where the index of refraction is the highest. Because the two orthogonally polarized incident waves of the optical beam travel through the birefringent material at different speeds, there will be a phase shift between the two waves when they emerge from the material. By choosing an appropriate thickness for the birefringent material, the required phase shift can be implemented. For the mid- and far-IR wavelength applications, however, birefringent materials are expensive and are not heat resistant for high optical powers due to residual absorption. This limits their applicability in high power applications. Additionally, because the dimensions of the phase retarder scale with the beam size of the impinging optical beam, the result is very expensive and large devices are required to typical application beam sizes.
Prior Art 180 Degrees Phase Retarders
One common high power laser application is in the field of laser machining and laser cutting. In such applications, a high power, focused laser beam is used to cut or scribe a work piece (typically a metal work piece, although laser cutting is also employed with plastics, paper, and other materials). It is well known in laser cutting applications that the cutting profile or width of the cut produced by powerful laser radiation (e.g. CO2) optical radiation at xcex=10.6 xcexcm) depends on the polarization orientation of the beam with respect to the cutting direction. This phenomenon is illustrated in FIG. 2a. As shown, the cutting width 12 is widest when the cutting direction is aligned with the polarization orientation of the cutting beam, as represented by orientation indicator 15. By contrast, the cutting width 11 is narrowest when the cutting direction is parallel to the polarization of the beam, as indicated by 14, and the cutting width 13 is intermediate when the polarization orientation is at some acute or obtuse angle to the cutting direction, as indicated by 13.
It is known in the prior art that uniform cutting results can be obtained when the direction of polarization of linear polarized light kept parallel to the cutting direction. This is illustrated in FIG. 2b where tie cutting width 16, 17, 18 is uniform because the polarization orientation of the incident light 19, 20, 21 is maintained parallel to the cutting direction. Such a system requires that the polarization direction of the cutting beam is dynamically aligned, i.e. rotated, during the cutting process. This requires that the phase retarder can be rotated. One prior art solution to the need to rotate the phase retarder is the use of mirrors to rotate the entire optical system around its optical axis. Such a system is disadvantageous because of the large size required for the optical set up. Furthermore, the need for critical alignment of the mirrors requires that the optical system be mechanically isolated from vibration during operation. These disadvantages add to the cost of the cutting tool.
A preferable approach to aligning the polarization orientation of a high power laser beam is through phase shifting of the polarization components. One such prior art approach is illustrated in FIG. 3. A transmissive type phase retarder is illustrated comprised of a multi-layer 35 coated phase shifting plate 30. Use of the illustrated half-wavelength plate or a combination of two quarter-wavelength plates allows rotating the plane of polarization of a light beam very effectively, as taught by Born and Wolf in Principles of Optics. London, England, 1975 at pp. 52-59. The theory and fabrication methods of plates with multi-layer coatings 35 are well known. See, e.g. W. H. Southwell, xe2x80x9cMultilayer Coating Design Achieving a Broad Band 90xc2x0 Phase Shift,xe2x80x9d Appl.Opt., 8-1980, pp. 2688-2692; T. N. Krylova, xe2x80x9cThe Reflection of Light from a Coated Surface at Various Angles of Incidence,xe2x80x9d Soviet J. Opt. Tech., 1968, vol. 11, No 12, pp. 695-698; U.S. Pat. No. 4,536,063 to Southwell, xe2x80x9cTransmissive Phase Retarder.xe2x80x9d A disadvantage of the illustrated prior art phase retarder is the necessity of their orientation at some angle 37 with respect to the incident laser beam 32. This angle of incidence causes a displacement 34 of the light beam 33 as it passes through transmissive plate 30, causing an undesired displacement in the propagation path of the incident beam 32. This displacement must be compensated for by means of additional optical elements, thus making the total system more complicated and increasing the system""s dimensions along the propagation direction of light. When a dynamic rotation of the tilted multilayer-coated phase retarders is needed for the optimal cutting application discussed above, it is obvious that the propagation path of the beam is continuously changing, thus requiring a highly complicated optical system to compensate for the propagation path deviations introduced by the illustrated phase retarder. Another disadvantage arising from the inclined orientation of the multi-layer coated transmissive phase retarder illustrated in FIG. 3 is that the active area 31 of the device increases with the angle due to the projection principle. In other words, as the angle 37 increases, the beam 32 is incident across a larger surface of the retarder, thus requiring a larger element.
The prior art phase retarders have several disadvantages. Both reflective and transmissive phase retarders introduce lateral displacements into the propagation path of the incident beam (by reflection for reflective types and because of the angle of incidence and refraction for transmissive types). Additionally, angled retarders must be larger than the beam size to handle the larger diameter that is projected across the incident surface. Careful selection of materials is required because of the need to minimize residual absorption of the optical beam""s energy.
What is lacking in the prior art is a highly efficient phase retarder having a large transmission coefficient, appropriate for high power and large laser beam applications, that can be removed from the application without affecting the propagation path of the laser and that is insensitive to lateral displacements or vibrations. Furthermore, rotation of the device should rot affect the propagation path of the laser beam. These and other shortcomings in the prior art are overcome by the preferred embodiments of the present invention described in detail below.
The present invention provides optical components that are advantageous over the optical components of the prior art. One application where the present invention has particular use is in high optical power, large beam laser material processing (e.g. with CO2 lasers), although the invention ma y be embodied for use in other applications as well. The traditional elements for controlling the polarization state do not satisfy industrial application needs due to large material or large manufacturing costs of the components. The set of industrial-proof optical components is hence limited. An advantageous feature of the preferred embodiments of the present invention is that widely used and available material can be used for implementation of the folded phase retarders, which are suitable for industrial applications.
In one aspect the invention provides for a transmissive phase retarder for imposing a phase shift between the polarization components of an incident light beam. The phase retarder comprises a first substrate having an input surface having a plane oriented orthogonally to an incident light beam source, and a first periodically folded internal surface having a repeating triangular profile and having facets at an angle to the plane of the input surface. The device further includes a multi-layer coating adjacent the first internal surface comprising a plurality of first layers having a first refractive index alternative with a plurality of second layers having a second refractive index. The multi-layer coating imposes a phase shift between the polarization components of light transmitted through it. The device further includes an output surface having a plane oriented parallel to the plane of the input surface.
In another aspect, the invention provides for a transmissive phase retarder comprising a first element having a flat input surface and a first periodically folded output surface, a second element adjacent said first element and having a first periodically folded input surface aligned with and complementary in shape to said first periodically folded output surface of said first element and having a first periodically folded output surface, and a third element adjacent said second element and having a second periodically folded input surface aligned with and complementary in shape to said first periodically folded output surface and having a second flat output surface aligned in parallel with the flat input surface. The device further includes a first anti-reflection composite phase shifting coating interposed between said first and second elements and being comprised of alternating layers of first and second material, the first material having a high refractive index relative to the second material and a second anti-reflection composite phase shifting coating interposed between said second and third elements and being comprised of alternating layers of said first and second material. The device provides the advantageous feature that the first and second periodically folded output surfaces and the first and second periodically folded input surfaces having facets oriented at an angle with respect to the first input surface and the flat output surface.
In yet another aspect, the invention provides for a 21a method for converting a polarized light beam having one spatially independent polarization state into a polarized light beam having another spatially dependent polarization state. The method includes directing a laser beam orthogonally to a flat surface of a phase retarder, passing the laser beam through a first phase retarding multi-layer structure coated on a first component of the phase retarder having triangular cross-sections oriented along a predefined path, introducing a spatially dependent phase shift and introducing a first lateral displacement of the laser beam and passing the laser beam through a second phase retarding multi-layer structure coated on a second component of the phase retarder having triangular cross-sections oriented along a predefined path, which is complementary with the first triangular cross-sectional component, introducing an equal and additional phase shift and providing a second lateral displacement of equal magnitude as, but in an opposite lateral direction to, the first lateral displacement. In other aspects, the method further includes; passing the laser beams through an anti-reflection coating prior to the step of passing the laser beam through the first phase retarding multi-layer structure, and passing the laser beam through a second anti-reflection coating subsequent to passing the laser beam through the second phase retarding multi-layer structure.
An advantageous features of the preferred embodiments of the present invention is that one can insert the phase retarder into an optical setup (such as a laser cutting tool) without affecting the propagation paths of the visible and invisible laser beams. The laser beams are orthogonally incident and emergent such that alignment of the device is straightforward. A further advantageous feature is that the device does not have an optical axis and therefore as long as the beam enters the device orthogonally, vertical or horizontal displacements due to vibrations will not affect the operation. Advantageously, when the preferred embodiment phase retarder is rotated with respect to is axis of symmetry, the propagation path remains undisturbed. A yet further advantageous feature of the invention is that it can be embodied in a novel phase retarder having a reduced overall thickness and diameter. This provides for reduced costs for the preferred embodiments. Another advantageous feature is that the preferred embodiment folded phase retarding element occupies substantially less space along the propagation direction of the laser beam than prior art devices.